Since the creation of the first submarines, and not until the introduction of more revolutionary designs later in WW II and in the post-war period, submarines were made up of three distinct components: inner hull, outer hull, and superstructure.
The ARGO class had two hulls. One, internal, was divided into three sections; one in the middle and perfectly cylindrical, and two at the extremities, cuneiform in shape and decreasing in diameter to the ends which were in the form of semispherical bulbs. The internal hull was made of steel with low nickel content and 14.5 mm in thickness (diminishing to 14 mm at the extremities). These steel plates were riveted to cylindrical beams (reinforcement rings) spaced between 520 mm and 550 mm. The plates were double riveted and positioned in a brick layer fashion (each plate tended to overlap the other half of the adjacent plates.
The external hull, 6 mm in thickness, enclosed the fuel and water tanks, asset tanks, and oil tanks. Internally, the submarine was divided into six compartments:
Light structures made up the bow and stern, aft and forward of the hull, giving the submarine a flat deck, a cruiser-like bow and a round stern. This part was usually completely open to the ocean and seawater drained through wide openings above the outer hull.